Direct injection engines may provide increased performance so that engine efficiency may be improved. Directly injecting fuel into a cylinder can reduce temperature in a cylinder so that more air and fuel may be drawn into the cylinder. However, the air-fuel mixture within the cylinder may not be fully vaporized at the time of ignition at higher engine speeds and loads since there is less time to mix air with the fuel. Consequently, a portion of injected fuel may not completely oxidize, thereby forming carbonaceous soot within the cylinder. After the soot is expelled from the engine, the soot may be stored in a particulate filter for subsequent oxidation.
Some internal combustion engines employ a particulate filter or PF (e.g., gasoline particulate filter, GPF, or diesel particulate filter, DPF, based on the engine fueling configuration) in an exhaust system to trap particulate matter flowing through the exhaust system and thereby meet emission standards. However, if the PF is not periodically cleaned or regenerated, the accumulated particulate matter on the PF may cause an increase in the exhaust system backpressure which may further lead to decreased engine performance.
In order to periodically regenerate or purge the PF of particulate matter, measures may be taken to increase the exhaust gas temperature to above a predetermined temperature (e.g., above 600° C.) to incinerate the carbon particles accumulated in the filter. In some cases, PF may reach a high enough exhaust temperature during normal vehicle operation to passively perform a particulate filter regeneration. However, due to the location of the PF in the exhaust system (disposed downstream of an oxidizing catalyst (e.g. three way catalyst)), it may take longer than is desired for temperatures in the exhaust system to reach the PF so that regeneration may be initiated.
One example approach for expediting GPF regeneration is shown by Ulrey et al. in U.S. Pat. No. 9,394,837. Therein, in response to a tip-out condition, particulate filter regeneration may be initiated via ceasing to deliver spark while continuing to supply fuel to one or more cylinders. By ceasing spark delivery, the injected fuel may be ejected from the cylinders into the exhaust system where it may oxidize closer to the particulate filter, increasing the temperature of the particulate filter. Further, the engine air-fuel ratio may be adjusted to vary a state of the exhaust gas mixture entering the particulate filter. Specifically, the air-fuel ratio may be oscillated between a leaner than stoichiometric air-fuel ratio, for generating excess oxygen at the GPF, and a richer than stoichiometric air-fuel ratio, for generating excess fuel to react with the excess oxygen at the GPF.
However, the inventors herein have recognized potential issues with such systems. As one example, the exothermal effect of the air-fuel perturbation may vary based on the amount of air flowing through the engine. For example, more heat may be generated by the air-fuel perturbation at higher airflow conditions as compared to lower airflow conditions. On the other hand, air flow to the engine may be limited by the throttle position which is determined as a function of the operator torque demand. In U.S. Pat. No. 9,394,837, for example, the air-fuel perturbation is performed during a deceleration fuel shut-off condition (DFSO) when operator torque demand and air flow is low. If the air flow is increased to enhance the exotherm, the excess torque may degrade drivability. If the air-fuel perturbation is performed while operator torque demand is elevated, a larger portion of the exhaust heat may be used to drive a turbine to meet the torque demand, resulting in less exhaust heat being available for filter heating. As a result, it may be difficult to balance using exhaust heat for turbine operation with using exhaust heat for particulate filter heating. As another example, relying on the occurrence of a DFSO to regenerate the GPF may limit regeneration opportunities. As a result, the GPF may not be sufficiently or timely regenerated. As yet another example, GPFs tend to be packaged in a location that are slow to heat and not conducive to rapid heating. Spark retard alone may not be sufficient to heat the GPF. In addition, spark retard based approaches may be difficult to isolate to one bank of the engine, especially in engines where one bank has the GPF packaged in a hot location while the other bank has a GPF packaged in a cold location due to packaging constraints.
In one example, the issues described above may be addressed by a method for an engine, comprising: generating an exotherm at an exhaust particulate filter while continuing to provide driver demanded torque by spinning a turbocharger compressor via an electric motor and concurrently operating engine cylinders with cylinder-to-cylinder air-fuel imbalance, the imbalance adjusted to maintain an overall stoichiometric exhaust air-fuel ratio. In this way, particulate filter (e.g., gas particulate filter (GPF)) regeneration initiation may be expedited through rapid heating while meeting operator torque demand.
As one example, responsive to filter regeneration conditions being met (e.g., when particulate filter soot load exceeds a threshold), but the temperature at the filter being insufficient for regeneration, an exotherm may be generated by operating engine cylinders with an air-fuel imbalance while increasing air flow to the engine by spinning a turbocharger compressor using electric assist. By operating the engine with cylinder-to-cylinder air-fuel ratio imbalance, such as with some cylinders operating lean and other cylinders operating rich, an exotherm may be generated near the particulate filter via mixing unburned fuel from the rich burning cylinders with excess oxygen from the lean burning cylinders. At the same time, an overall exhaust air-fuel ratio may be maintained at or around stoichiometry. To further enhance the exothermic effect of the imbalance, a waste-gate valve may be opened so that all the exhaust can be directed to the filter while bypassing a turbocharger turbine. At the same time, the turbocharger compressor may be operated using assistance from an electric motor so that operator torque demand can be met and air flow to the filter can be increased. In addition, any transmission downshifts may be delayed until the particulate filter has been sufficiently heated. If the imbalance is not sufficient to raise the filter temperature, extra air may be delivered to the exhaust passage via use of an exhaust air pump and extra fuel may be delivered to the exhaust passage via use of an exhaust fuel injector. Once the filter temperature is high enough, filter regeneration may be initiated by operating the engine lean. This may include disabling fuel at lower vehicle speeds to regenerate the filter, opportunistically, during a DFSO. Alternatively, at higher vehicle speeds, the engine may be operated leaner than stoichiometry to provide extra oxygen to the exhaust passage while the exhaust injector is used to inject fuel in proportion to the extra oxygen to expedite soot burn-off at the filter.
In this way, particulate filter regeneration may be expedited by maintaining a higher filter temperature. By enhancing the exothermic effect of an air-fuel ratio imbalance using higher air flow provided via a turbocharger compressor, the time required to bring a loaded particulate filter to operating temperature is reduced, without degrading drivability or engine performance. By relying on electric assistance to drive the turbocharger compressor to meet torque demand, the turbocharger may be operated with a waste-gate valve fully open, enabling a larger portion of the heated exhaust to be directed to the filter, while bypassing the turbine. By using an exhaust air pump and fuel injector to generate exhaust heat at the particulate filter, the need for extended lean engine operation is reduced, improving exhaust NOx emissions. By expediting filter heating, filter regeneration can be performed more frequently, improving engine emissions performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.